1. Field of the Invention
The present invention relates to an apparatus for accurately and quickly measuring absorbed dose distribution in a phantom that is used in the course of a radiotherapeutic treatment of cancer using X-rays, particle beams such as electron beams or the like.
2. Description of the Prior Art
In intensity modulated radiotherapy (IMRT), a radiotherapeutic treatment, it has been a general practice to irradiate a patient by changing the field profile of radiation and, also, the direction of incidence of radiation so that the cumulative absorption dose of the radiation may be proportionated to an affected zone of the patient. Accordingly, IMRT has been recognized having merit in that irradiation can be concentrated on an affected zone to enhance radiotherapy.
However, when it comes to performance of the IMT, a treatment plan has to be set up in which irradiating conditions required for the affected zone to be irradiated in a predetermined absorption dose distribution are to be formulated. The validity of the treatment plan, however, requires experiment-based verification or evaluation and, for this purpose, the absorption dose measuring apparatus for IMRT (hereinafter referred to as “IMRT-dedicated absorption dosimeter”) is generally utilized.
Measurement of the absorption dose distribution performed by the conventional IMRT-dedicated absorption dosimeter will be described hereinafter with reference to FIG. 14. As shown in FIG. 14, the conventional IMRT-dedicated absorption dosimeter includes a plurality of phantoms 102 and a corresponding number of X-ray films 103 respectively sandwiched between neighboring phantoms 102. With this conventional IMRT-dedicated absorption dosimeter, the phantoms 102 are irradiated with radiation 101a and 101b such as particle beams, X-rays, electron beams, or the like and, as a result thereof, the X-ray films 103 sandwiched between neighboring phantoms 102 are exposed due to the interaction with the radiation 101a and 101b. By measuring a distribution of the optical density in the X-ray films 103, a two-dimensional absorption dose distribution at various positions on the X-ray films 103 can be obtained. Then, a three-dimensional absorption dose distribution within the phantoms 102 is obtained based on the two-dimensional absorption dose distribution so measured with respect to all of the X-ray films 103.
Radiosurgery is also known in the art as an alternative choice of the radiotherapeutics. This Radiosurgery makes use of a slender beam of a diameter with a few cm2 of the area of the radiating field. Hereinafter, measurement of the absorption dose distribution with the radiation dosimeter for the Radiosurgery will be described with reference to FIG. 15. As shown in FIG. 15, measurement with the radiation dosimeter for the Radiosurgery requires the use of a water phantom 111 in which a microchamber 112 is inserted so that the microchamber 112 can be scanned within the water phantom 111 in a direction shown by the arrow Y to measure a distribution of absorbed dose.
However, the IMRT-dedicated absorption dosimeter utilizing the X-ray films as shown in FIG. 14 has a problem in that since the absorption characteristic of the X-ray films differs considerably from that of a human body, accurate measurement of the absorption dose pattern is difficult to achieve. Also, depending on the manufacturing lots and/or developing conditions of the X-ray films, the output tends to vary even though those X-ray films have absorbed an equal amount of radiation, resulting in difficulty in achieving measurement accuracy. Also, the work of developing the exposed X-ray films and measuring the optical density requires a substantial amount of labor and a substantial amount of time.
In the practice of the IMRT, an irregular radiation field is defined with the use of a multi-leaf collimator. In such case, the edge of the resultant radiation field has a generally wedge-shaped configuration not larger than a few millimeters and requires the use of a measuring instrument having a spatial resolving power not larger than a few millimeters if an accurate distribution of radiated dose is to be measured. Similarly, even measurement of the absorbed dose distribution for Radiosurgery requires the use of a measuring instrument having a spatial resolving power not larger than a few millimeters so that a beam used to irradiate an extremely small radiation field is to be measured. However, scanning within the water phantom of the microchamber having a spatial resolving power not larger than a few millimeters requires a substantial amount of labor and a substantial amount of time. Yet, there is an additional problem in that since the sensitive region is so small, the output current is so low as to result in a low signal-to-noise (S/N) ratio.